1. Field of the Invention
This invention relates to signal processing apparatus and methods. In particular the invention relates to apparatus for, and methods of digitizing digitally modulated data which has subsequently been demodulated, as are used in cordless telephone systems.
In a communication system where frequency shift keying (FSK) is used as the method of modulation for the transmitted digital data, the received modulated signals are first demodulated and then redigitized.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a known apparatus for digitising demodulated FSK modulated data.
Referring to FIG. 1, a comparator 1 is used to compare the instantaneous value of the incoming demodulated signal I with its mean level. The mean level is determined by filtering the signal I with a low pass filter comprising a resistor R and capacitor C, the filter having a long time constant compared to the bit period of the data, the time constant being determined by the values of the resistor R and capacitor C.
This simple process is, however, unsuitable for a system where the data is transmitted in bursts, such as in the second generation cordless telephone system, commonly known as CT2. For example in CT2, transmission and reception takes place in 1 ms bursts with the demodulated signal I having an arbitrary DC level during reception of the transmitted data. This will cause the mean DC level at the reference input of the comparator 1 to drift, making the detection of the data at the start of reception impossible. Similar problems will also arise when receiving data which is highly unbalanced; i.e. a long sequence of 1s or 0s. There exists another problem in applications where the transmitter and receiver centre frequencies are allowed to drift from their nominal values by specified amounts: this will also cause the mean level of the demodulated signals to drift.
Various solutions have been proposed to minimise the effect of the mean level of the demodulated signal drifting as a result of frequency offsets in the transmitted data, unbalanced data and the receiver being turned off during transmission. All of these solutions, however, have severe limitations, and fail to provide a complete solution to DC drift related problems.